Ganong's Review of Medical Physiology, 23rd Edition

354 SECTION IV Endocrine & Reproductive Physiology
anxiety, and apprehension cause marked increases in ACTH
secretion. Input from the suprachiasmatic nuclei provides the
drive for the diurnal rhythm. Impulses ascending to the hypothalamus
via the nociceptive pathways and the reticular formation
trigger increased ACTH secretion in response to
injury (Figure 22–18). The baroreceptors exert an inhibitory
input via the nucleus of the tractus solitarius.
GLUCOCORTICOID FEEDBACK
Free glucocorticoids inhibit ACTH secretion, and the degree
of pituitary inhibition is proportional to the circulating glucocorticoid
level. The inhibitory effect is exerted at both the pituitary
and the hypothalamic levels. The inhibition is due
primarily to an action on DNA, and maximal inhibition takes
several hours to develop, although more rapid “fast feedback”
also occurs. The ACTH-inhibiting activity of the various steroids
parallels their glucocorticoid potency. A drop in resting
corticoid levels stimulates ACTH secretion, and in chronic adrenal
insufficiency the rate of ACTH synthesis and secretion is
markedly increased.
Thus, the rate of ACTH secretion is determined by two
opposing forces: the sum of the neural and possibly other
stimuli converging through the hypothalamus to increase
ACTH secretion, and the magnitude of the braking action of
glucocorticoids on ACTH secretion, which is proportional to
their level in the circulating blood (Figure 22–19).
The dangers involved when prolonged treatment with antiinflammatory
doses of glucocorticoids is stopped deserve
emphasis. Not only is the adrenal atrophic and unresponsive
after such treatment, but even if its responsiveness is restored
by injecting ACTH, the pituitary may be unable to secrete
normal amounts of ACTH for as long as a month. The cause
of the deficiency is presumably diminished ACTH synthesis.
Thereafter, ACTH secretion slowly increases to supranormal
levels. These in turn stimulate the adrenal, and glucocorticoid
output rises, with feedback inhibition gradually reducing the
elevated ACTH levels to normal (Figure 22–20). The complications
of sudden cessation of steroid therapy can usually be
avoided by slowly decreasing the steroid dose over a long
period of time.
EFFECTS OF
MINERALOCORTICOIDS
ACTIONS
Aldosterone and other steroids with mineralocorticoid activity
increase the reabsorption of Na + from the urine, sweat, saliva,
and the contents of the colon. Thus, mineralocorticoids
cause retention of Na + in the ECF. This expands ECF volume.
In the kidneys, they act primarily on the principal cells (P
cells) of the collecting ducts (see Chapter 38). Under the influence
of aldosterone, increased amounts of Na + are in effect
Trauma via
nociceptive
pathways
Afferents
from NTS
CRH
FIGURE 22–19 Feedback control of the secretion of cortisol
and other glucocorticoids via the hypothalamic-pituitary-adrenal
axis. The dashed arrows indicate inhibitory effects and the solid
arrows indicate stimulating effects. NTS, nucleus tractus solitarius.
Plasma concentration
Systemic
effects
Cortisol
High
Normal
Low
0
ACTH
ACTH
FIGURE 22–20 Pattern of plasma ACTH and cortisol values
in patients recovering from prior long-term daily treatment with
large doses of glucocorticoids. (Courtesy of R Ney.)
exchanged for K + and H + in the renal tubules, producing a K +
diuresis (Figure 22–21) and an increase in urine acidity.
MECHANISM OF ACTION
Anterior
pituitary
Emotion via
limbic system
Drive for
circadian
rhythm
Hypothalamus
Adrenal
cortex
Cortisol
2 4 6 8 10 12
Months after stopping
glucocorticoid treatment
Like many other steroids, aldosterone binds to a cytoplasmic
receptor, and the receptor-hormone complex moves to the